Sensor arrays are commonly used for diagnostic imaging, in medical and scientific applications. Conventionally these have outputs which may be visual, resulting in a film image, or digital, resulting in electrical data for transmission to a suitable data processing unit. In order to be useful, good spatial and energy resolutions are required, i.e. the number of pixels per unit area resolved and the sensitivity to the expected energy spectrum, must be coincident with the task requirements.
A 1-D digital detector array is taught in U.S. Pat. No. 4,303,860 (Bjorkholm et al), wherein a narrow fan beam of X-rays is passed through an object to be imaged and is received by a scintillation crystal which generates light, in proportion thereto. The light, in turn, is received by an array of photodiodes, in itimate contact with the crystal, which generate a current in proportion to the intensity of illumination. The current may be detected by any suitable means, such as an array of CCDs or other sensor. Relative movement between the object and the radiation source with its detector array is required in order to completely image the object.
By using a two-dimensional detector array, a single divergent X-ray beam may be directed through an object onto a large area panel, which may be of any arbitrary size. In U.S. Pat. No. 4,626,688 (Barnes) FIGS. 1 and 2 of U.S. Pat. No. 4,707,608 (DiBianca) both one-dimensional and two-dimensional arrays are shown. As in Bjorkholm et al, Barnes discloses a radiation sensitive material responsive to incident radiation to produce light and an adjacent array of photodiodes responsive to the light to produce an analog electrical signal indicative of radiation intensity. In DiBianca primary radiation (X-rays) produces secondary radiation (electron-ion pairs) within a detector which, in turn, produces charge carries in a detector.
Recent progress in amorphous silicon fabrication techniques has enabled this material to be deposited and patterned over large areas so as to create page size integrated electronics. Typically, for most electronic applications, as well as for optical sensing applications, this material is no more than a few microns thick. When used with a radiation detecting phosphor screen, X-rays impacting the phosphor cause it to emit visible light which will generate a current normally through the adjacent amorphous silicon detector layer which, in turn, must be picked up by a suitable detection circuit. A major shortcoming of devices of this type is their extremely low detection efficiency. Because the phosphor screen typically has an efficiency generally below 10% in generating light in response to incident radiation, even with a highly efficient optical sensor coupled thereto, devices of this type can be no more than about 10% efficient.
While thin amorphous silicon layers are highly efficient in detecting visible light, it is known that thick amorphous silicon layers (on the order of 10 to 50 .mu.m) may detect high energy radiation (i.e. radiation with high energy photons or particles) with an efficiency of greater than 90%. In U.S. Pat. No. 4,785,186 (Street et al) a thick amorphous silicon layer in a p-i-n doped configuration with electrically biased upper and lower metal electrodes will pass current therethrough, thereby detecting the existence of high energy radiation. Particle position detection can be accomplished by an array of such devices connecting the output currents to the input of amplifiers. The direct detection of radiation induced electron-hole pairs passing through the detector layer is much simpler than the arrangement afforded by scintillation systems in which the high energy radiation is converted to visible light which is detected by an adjacent sensor.
In a paper entitled "A New Type of Field-Effect Image Storage Panel with a Photoconductive Charging Layer" (Kazan et al), Proceedings of the IEEE, Vol. 56, No. 11, pp. 2057-2059, November 1968 there is taught an electroluminescent imaging panel in which local charging or discharging of an adjacent semiconductor layer (ZnO) is controlled by a next adjacent photoconductive layer. Thus, in areas irradiated by an image of UV radiation, the resultant currents through the photoconductive layer produce trapped charges at the interface with a ZnO layer which, in turn, controls the local conductivity of the ZnO layer. In accordance with these variations in conductivity, local currents are allowed to flow through an electroluminescent phosphor across which voltage is maintained. A stored visible image is thus produced by the electroluminescent phosphor layer, corresponding to a transient image.
A transistor-photodiode array with multiplexed scanning is taught in "Advances in Image Pickup and Display", Vol. 6 pp. 184, 185, .COPYRGT.1983, in the chapter entitled "Image Sensors for Television and Related Applications" (Weimer et al). As schematically illustrated in FIG. 4, each pixel location is provided with a photosensor and a transisor.